The present invention relates to communications, and in particular to a trellis decoder.
Broadband modems, and in particular asymmetric digital subscriber line (ADSL) modems dramatically increase the ability to transfer data over conventional telephone lines. Significantly, ADSL modems allow data transfers at rates over two hundred times faster than conventional modems, and over ninety times faster than ISDN lines.
The bandwidth of a conventional copper twisted pair telephone line is approximately 1 MHz. However, conventional analog signals that carry voice over these lines operate in a bandwidth that is only 4 kHz wide. Advantageously, ADSL takes advantage of the remaining portion of the 1 MHz. Specifically, ADSL technology effectively subdivides the 1 MHz bandwidth of the copper twisted pair line into three information channels: i) a high speed down stream channel, ii) a medium speed duplex (upstream/downstream) channel, and iii) a conventional voice channel. Downstream refers to transmissions from the telephone network to the ADSL modem located at a subscriber site, while upstream is the route from the subscriber site to the telephone network. This multichannel approach enables subscribers to access the internet, order a video for viewing and send a facsimile or talk on the telephone all at the same time.
To ensure commonality of the various ADSL modems that will be deployed and the telephone central office (CO), industry has been working with the American National Standards Institute, Inc. to establish a standard for the interface between the ADSL modems and the telephone CO. This standard is designated T1.413 and entitled xe2x80x9cInterface Between Networks and Customer Installationxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interfacexe2x80x9d. The standard specifies that the transmission encoders use constellation encoding. One type of constellation encoding is trellis encoding.
U.S. Pat. No. 4,980,897 entitled xe2x80x9cMulti-Channel Trellis Encoder/Decoderxe2x80x9d (hereinafter xe2x80x9cthe ""897 Patentxe2x80x9d) discloses a trellis encode/decoder. As shown in FIG. 11 of the ""897 Patent, the decode process includes the steps of initializing the decoder to a known state and then reading the received signal (Xn, Yn) from the receive vector buffer. Next, the nearest points from each coset are determined. The decoding process than performs maximum likelihood sequence estimation using the Viterbi algorithm.
The ""897 Patent discloses that the step of determining the closest coset points to the received point involves computing the Euclidean distance between each point in the constellation and the received point, and then comparing the distances. Significantly, as the number of bits in a received point signal increases, so does the number of points in the constellation, and thus the number of computations and comparisons that must be performed to determine the closest coset points. That is, as disclosed in the ""897 Patent each point in the constellation has to be compared to the received point, and thus the number of computations and comparisons is rather large. For example, if the signal has N bits, then 2N-1 comparisons are required (e.g., if N=15 then 32,767 comparisons are required). U.S. Pat. Nos. 5,301,209; 5,706,312; 5,519,731 and 5,530,707 also disclose various aspects of trellis encoding and decoding.
Therefore, there is a need to quickly and efficiently determine the closest coset points to the received point in the trellis decoder.
An object of the present invention is to efficiently identify the closest coset points to a received point in a trellis decoder.
Briefly, according to the present invention, a trellis decoder identifies the closest points for each coset in a trellis decoder by reading a received point and determining upper and lower threshold values in a signal constellation to define a decode region within the constellation. The dimensions of the decode region are based on the number of bits of information in the received signal. For a four dimensional trellis code, the decoder translates the received point in four directions to provide four image points. Any image points that would be outside the constellation decode region are mapped into the decode region to ensure that the four image points are within the decode region of the constellation. For each of the cosets, bit extraction is then performed to find the closest point to the received point.
Once the closest coset points are identified, the trellis decoder performs a maximum likelihood sequence estimation using the Viterbi algorithm to determine the received sequence.
Advantageously, the trellis decoder of the present invention provides a fast technique for determining the closest points to a received point for each coset. The decoder is preferably implemented as a state machine. However, the present invention may also be incorporated in a central processing unit having sufficient processing speed to support the communications that employs the trellis decoder.
These and other objects, features and advantages of the present invention will become apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.